The present invention concerns combined complementary filtering spectrum analysis.
It is used generally in spectrum analysis, and more precisely in RAMAN spectrometry.
In the document FR-A-2 681 941, the Applicants have already proposed a RAMAN spectrometry spectrum analysis device which comprises:
a source for producing excitation radiation of a predetermined wavelength; PA1 a location for a sample to be analyzed; PA1 means for guiding the excitation radiation onto a sample placed in the location; PA1 a separator filter for reflecting, in a first direction, a first spectral band of the radiation it receives and which transmits, in a second direction, the remainder of the radiation which it receives, the first spectral band being centred on the wavelength of the excitation radiation and the remainder of the radiation transmitted by the separator filter corresponding to the desired RAMAN spectrum; PA1 means for collecting the radiation scattered by the excited sample, and for guiding it onto the separator filter; PA1 spectral detection and analysis means disposed in the second transmission direction; and PA1 means for collecting the RAMAN spectrum transmitted by the separator filter, and guiding it onto the detection and analysis means. PA1 a band-eliminating filter with a steep eliminating slope, which filter can reject a second spectral band which is narrower than the first spectral band, and embraces the wavelength of the excitation radiation; PA1 means for collecting the first spectral band of the radiation reflected by the separator filter, and for guiding it onto the band-eliminating filter in order to eliminate the second spectral band therefrom; and PA1 means for collecting the radiation coming from the band-eliminating filter and for guiding it onto optical means which combines it, in the second direction, with the RAMAN spectrum transmitted by the separator filter, such that the detection and analysis means process the complementary radiation combined in this way. PA1 a first rigid leg mounted so as to slide in translation through an aperture in the support of the separator filter mounted such that it is fixed in translation; and PA1 a second rigid leg bearing the compensator component;
Here, the separator filter separates the polychromatic radiation scattered by the sample into transmitted radiation and reflected radiation.
In practice, the transmitted radiation comprises the useful spectrum which is characteristic of the sample to be analyzed. For example, in RAMAN spectrometry, the useful spectrum is the so-called. "RAMAN" spectrum resulting from the wavelength changing effect accompanying the scattering of the light by the sample when the latter is illuminated by monochromatic excitation radiation. In contrast, the reflected radiation is restricted to a spectral band centred on the wavelength of the excitation radiation corresponding to the RAYLEIGH scattering line without a wavelength change.
Most frequently, the separator filter is of the type which rejects a relatively narrow spectral band ("notch"), such as dielectric multilayer interference filters, so-called ASHER colloidal filters, or holographic filters. This type of filter is advantageously easy to use and has a very high transmission coefficient outside the band to be eliminated.
However, the disadvantage of this type of filter is that it has a relatively flat elimination slope (non-steep edges) and a rejection or elimination band which is relatively wide in comparison with the excitation line to be eliminated.
The result of the above is that useful data, which is characteristic of the sample to be analyzed and known to the person skilled in the art as "low frequency lines", is not transmitted by the separator filter to the detection and analysis means but is eliminated (reflected) with the excitation radiation, which results in a significant loss of data.
A known solution is to use the separator filter as a high-pass or low-pass filter, set at the wavelength of the excitation line. However, the disadvantage of such a solution is that it eliminates either the STOKES part or the ANTISTOKES part of the RAMAN spectrum.
Another solution would be to use a narrow band-eliminating-type separator filter having a steep eliminating slope. However, such a filter is not at present technically viable.